1. Field of the Invention
The present invention relates to a method and apparatus to control the fuel/air mixture of a diesel engine and, more particularly, to a method and apparatus to control the fuel and air ratio of a diesel engine during a load increase.
2. Description of the Related Art
In modern low-emission diesel engines, the fuel/air mixture is typically set lean of the stoichiometric level with exhaust gas recirculation (EGR) used to reduce NOx during steady state operation. During rapid load increases on turbocharged diesel engines, the air flow increase lags behind the fuel flow increase and results in relatively rich operating conditions. This results in increased smoke and particulate emissions. Typically, the EGR flow is reduced or eliminated during rapid load increases to reduce smoke and particulates. However, this causes high NOx emissions from the engine.
In internal combustion engines, EGR is a NOx emission reduction technique used in most gasoline and diesel engines. EGR works by recycling a portion of an engine's exhaust gas back to the engine cylinders. Often, the EGR gas is cooled through a heat exchanger to allow introduction of a greater mass of the recirculated gas into a diesel engine. Since diesel engines are typically unthrottled, EGR does not lower throttling losses in the way that it does for gasoline engines. However, the exhaust gas, which is largely carbon dioxide and water vapor, has a much lower oxygen mass fraction than air, and so it serves to lower peak combustion temperatures. There are tradeoffs, however, adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that correspond to an increase in EGR. Particulate matter, which may mainly be composed of carbon, but is not burned in the power stroke is wasted energy.
Usually, an engine recirculates exhaust gas by piping it from the exhaust manifold to the inlet manifold. A control valve (EGR valve) within the EGR circuit regulates the time and the amount of return flow.
The air/fuel ratio is the mass ratio of air to fuel present during combustion. When all of the fuel is combined with all of the free oxygen, typically within a vehicle's combustion chamber, the mixture is chemically balanced and this air/fuel ratio is called a stoichiometric mixture. In theory, a stoichiometric mixture has just enough air to completely burn the available fuel. In practice, this is never quite achieved, due primarily to the very short time available for the combustion in an internal combustion engine for each combustion cycle.
What is needed in the art is a method and an apparatus to reduce pollutants during an increased torque requirement transition for diesel engines.